Corona discharge treatment (CDT), which produces a gaseous discharge at atmospheric pressure, is widely used to promote the adhesion of photographic layers to the supporting base material or to one another. Examples of the use of corona discharge treatment are found in U.S. Pat. Nos. 4,394,442; 4,229,523; 4,363,872; 4,292,032; 4,663,263; 4,009,037; 4,087,574; 4,055,685; 5,004,669; 3,874,877; 3,860,427; 3,630,742; 3,607,345; 3,531,314 and 3,411,908. Corona discharges, however, are relatively uncontrolled. Most often, they take place in an ambient environment, so that differences in relative humidity and temperature can lead to non-reproducible results. Another characteristic of corona discharge treatments at atmospheric pressure is the presence of streamers, or intense localized discharges. We have found, by using x-ray photoelectron spectroscopy (XPS), that surfaces of CDT treated polymers have large spatial variations in the degree of treatment. It is theorized that the variability of the surfaces is due to the presence of streamers.
To avoid these problems, the use of glow discharge treatments (GDT), which take place at reduced pressures, has been suggested. A GDT discharge is much more diffuse than a CDT discharge and the treatment is uniform and reproducible. This makes it a better process than CDT for many applications. Glow discharges are a well-known method of promoting adhesion in a wide variety of systems. For example, U.S. Pat. No. 3,288,638 describes the GDT treatment of polymers in an inert gas environment to promote adhesion of metals. The desirable results of adhesion are attributed to the bombardment of the surfaces by inert gas ions and electrons.
U.S. Pat. No. 3,837,886 describes the use of GDT to promote the adhesion of a gelatine containing binder to biaxially drawn, surface roughened polystyrene. Tatsuta and Ueno, the inventors of U.S. Pat. No. 3,837,886, find that GDT is ineffective without first roughening the polystyrene surface. They also report that the choice of treatment gas is unimportant.
Dolezalek, et al (U.S. Pat. No. 4,451,497), discuss the treatment of various polyester supports to promote the adhesion of photographic emulsions directly onto the support. They attribute the beneficial effects to a reduction in surface charge and report no particular dependence on the gas used in the treatment.
Ishigaki and Naoi (U.S. Pat. No. 4,933,267) report using GDT in connection with improving the dimensional stability of photographic systems, but mention no reason to choose a particular treatment gas. The implication is that the treatment is done in residual air. In a subsequent patent (U.S. Pat. No. 4,954,430), they describe the use of GDT prior to coating with a copolymer containing vinylidine chloride and again fail to mention any advantage to the use of a particular treatment gas.
Stroebel (European Application 92303556.2) teaches that nitrogen CDT can be effective, carried out at atmospheric pressure, if the temperature of the support is elevated during treatment. There is no evidence that GDT in nitrogen at ambient temperatures would be effective. Thus, the prior art does not teach what surface conditions are required on a polyester substrate for promoting adhesion of photosensitive coating compositions.
Finally, in U.S. Pat. No. 5,324,414 filed on Jun. 1, 1992 an Ion Selective Electrode and Method of Manufacture is described. The method uses glow discharge to deposit metallic silver on a polymeric surface. This application also does not teach what surface conditions are required for promoting adhesion of photosensitive coatings to polyester substrates.
The present invention characterizes the surface chemistry of the support that is beneficial for adhesion of the photographic layers. The treatment conditions for obtaining a proper support using GDT, are optimized. The optimization includes the type of gas in which the treatment takes place, the pressure and the energy density.